A time-domain reflectometer (TDR) is an electronic instrument that uses time-domain reflectometry to characterize a conductor or any other electrical transmission path. A TDR measures reflections along a conductor, such as a transmission line on a printed circuit board (PCB). In order to measure such reflections, the TDR transmits an incident electrical signal onto the conductor and measures a corresponding reflected signal. If the conductor is of a uniform impedance and is properly terminated, then there will be no reflections and the remaining incident electrical signal is absorbed at the far-end conductor termination point. Instead, if there are impedance variations on the conductor, then some of the incident electrical signal is reflected back to the source.
The reflection coefficient is a parameter that describes how much of the electrical signal is reflected by the impedance variation in the electrical transmission path. The reflection coefficient is equal to the ratio of the amplitude of the reflected signal to the incident signal. For example, in an electrical transmission line the reflection coefficient provides a measure of how much an electrical signal is reflected by an impedance.
The TDR measures the reflection coefficient at a given test point and calculates the characteristic impedance according to the measured reflection coefficient. The TDR is commonly used to determine the characteristic impedance of a transmission line, or interconnect, on a PCB. The characteristic impedance is measured to verify the proper manufacturing of the transmission lines on the PCB. In high performance PCBs, the transmission line interconnects must have a proper characteristic impedance Z0, and the characteristic impedance Z0 should be a specific value, such as 50 ohms, depending on the particular design. The TDR measures the reflection coefficient ρ and calculates the corresponding characteristic impedance Z0 of the transmission line according to the conversion formula:Z0=50(1+ρ)/(1−ρ)  (1)The constant value “50” in equation (1) may be different depending on the application. For example, the characteristic impedance of a differential transmission line can be calculated by replacing the constant value “50” with “100”.
A PCB is made on a panel that also includes “coupons” which are conductive traces on the surface of the panel, made to mimic the transmission lines, where the characteristic impedance measurements are made. In the manufacturing environment, when the PCB is built, the characteristic impedance is measured at one or more coupon locations, the measurement being made by a TDR.